During the summer of 2019, Oklahoma residents experienced mysterious tremors that swept through the state and collectively came to be known as “the anomaly.”
Scientists, puzzled by the pulses, ultimately tracked the signals down to a military facility that stores America’s largest ammunition stockpile and oversees huge explosions of obsolete munitions, a process that sends low-frequency acoustic waves called infrasound rippling across the landscape. In addition to providing a great science detective story, the infrasound waves produced by the routine explosions at McAlester Army Ammunition Plant have enormous potential to reveal unknown details about the sky, according to Stephen Arrowsmith, who serves as Hamilton Chair in Earth Sciences at Southern Methodist University and is among the scientists monitoring the Oklahoma pulses.
Infrasound waves produced by the munitions blasts not only travel across land, they also speed skyward into the stratosphere, a layer of the atmosphere that extends for dozens of miles above Earth’s surface. Researchers have been monitoring the refracted waves that bounce back from these high altitudes, a method that exposes elusive phenomena about the atmosphere that cannot be obtained through other means.
Arrowsmith will present the results of his team’s research—which has a range of applications from national security to weather reports—on Wednesday at the 182nd Meeting of the Acoustical Society of America in Denver, Colorado.
“One of the things that obviously generate infrasound are signals like this, where there are routine explosions happening,” Arrowsmith said, referring to the munitions plant, in a call. “Those are really useful because if the source is always the same, the differences [in infrasound waves] tell us about the atmosphere. That’s what we’re trying to do—use them as a way to probe the atmosphere.”
Infrasound waves reverberate below the range of human hearing, though some animals, including whales and elephants, are tapped into this acoustic band. The low frequencies of infrasound waves allow them to travel much further than audible sound waves, making them ideal tools to explore some of the most enigmatic yet consequential regions of the sky.
“Audible sound just doesn’t travel very far,” Arrowsmith explained. “You can’t hear people a long way away. But infrasound does travel really far and it doesn’t get absorbed by the gasses in the atmosphere nearly as much. You can use it to observe things that are on the other side of the world, if they’re big enough, so it’s quite useful for monitoring.”
Fortunately, there is an existing worldwide network of 60 infrasound stations that are designed to detect atmospheric nuclear tests, which are prohibited under the Comprehensive Nuclear-Test-Ban Treaty. In addition to Arrowsmith’s work in Oklahoma, scientists have used these detectors to capture new insights about the atmosphere by studying waves from munitions explosions in a Finland-based plant, as well as pulses from accidental explosions in Bulgaria and Albania.
The refracted waves that these teams study are imprinted with valuable information about the places in the atmosphere they’ve visited. Sometimes they interact with phenomena such as gravity waves, which are transient ripples in the sky that can impact weather patterns far below.
Gravity waves “transfer energy and momentum through the atmosphere all the way up into the stratosphere,” Arrowsmith said. “They are hard to measure with other other techniques and they are important because it’s recognized that what happens in the stratosphere does affect the weather that we experience near the ground in the troposphere, but our existing weather models don’t take that into account. They basically stop at about 35 kilometers above the ground—that’s the cap.”
“It’s thought that if we have better measurements of the stratosphere, then we should be able to better make predictions about our weather,” he added.
To that end, Arrowsmith and his colleagues plan to expand the network of sensors that they have already installed to monitor explosion-forged infrasound waves in Oklahoma. But he also notes that the atmospheric knowledge revealed by these waves is only one of many applications of infrasound: The waves can also be used to predict volcanic eruptions, or to monitor battlefield activities such as artillery fire or helicopter flights, or to detect explosions of extraterrestrial objects in the atmosphere. There are even space scientists who plan to fly infrasound detectors through the cloudy skies of Venus in order to probe its largely unexplored and poorly understood surface.
“That’s sort of what makes it fun,” Arrowsmith concluded. “We can really look at a whole bunch of different types of things—and we do.”
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